This invention relates to an image reproduction system and, more particularly, to a digital system for producing a halftone image reproduction of a continuous tone original image.
Halftone reproduction methods have heretofore utilized a technique in which a continuous tone photograph is reproduced through a halftone screen upon a high contrast photosensitive material. The screen may be made of two sheets of glass, each covered with black lines, and oriented so that the black lines on one are at right angles to the lines on the other. The screen permits light to pass only through the spaces between the lines so that the light is broken into tiny dots, the size of each of which is proportional to the amount of light reflected from the various tones in the original picture. The reflected light passing through the screen is photographed on a high contrast photosensitive material which is then developed, transferred to an engraving plate which is then etched by standard photo engraving techniques to produce a metal plate ready for printing. Another coventional and currently more popular camera screen halftone technique is to replace the ruled glass screen with a contact screen which allows for better brightness distribution. Both of these techniques require considerable amount of camera work artistry, opto-mechanical expertise and a great deal of processing labor in order to produce high quality halftone reproductions.
The advent of modern day electronic photocomposition systems in the printing and publishing industry has produced devices which are capable of high speed production of justified copy on photographic materials ready for flat engraving or paste up for letter press or lithographic reproduction. However, these photocomposers have limited capability and do not completely satisfy the need for high speed full page newspaper production, mainly because of the inability of these systems to include continuous tone graphics in their format. Much work has been done in an attempt to produce electronic halftones which are quality competitive with original camera-screened halftones and compatible with a text composing system. However, the various techniques proposed have yet to produce a halftone quality which is competitive with the camera-screening technique.